As much as fans of both pairings say there's canonical evidence to support their claim, a lot of it is left to subjective interpretation. I'm still a firm believer that Cloud could have been with either of them -- or neither given that um romance is not more important than saving the world. The fact that Cait Sith's only accurate prediction included Aerith dying and her/Cloud being a perfect match kind of sums that up for me, and it's hard to deny that Cloud had a thing for Tifa growing up (and even harder to deny that they had sex underneath the Highwind). Arguments on both sides are really intense and broad with lots of material from various other games, interviews, etc, but I just -- I don't think there is a right answer here and it's the speculation that interests me the most.